This invention relates to hotmelt adhesives for bonding DVDs of sandwich construction and to a process for the production of !DVDs of sandwich constuction
xe2x80x9cDVDxe2x80x9d is the abbreviation for digital versatile disc or digital video disc. These are optical storage media similar to the known CDs (compact discs). The principal difference between DVDs and CDs and other known storage media is the far higher density of the musical, graphic or data information stored in DVDs. This higher data or information density of the storage medium imposes stringent demands on the manufacturing process and the materials used therein.
One possible structure for a DVD is schematized in FIG. 1. The DVD in question is a so-called (DVD 5 which consists essentially of two halves,) is one-sided and carries a layer of information; it has a storage capacity of 4.7 gigabytes. In FIG. 1, the layer which carries the information is denoted by the reference numeral xe2x80x9c1xe2x80x9d while the outer layer which does not carry any information is denoted by the reference numeral xe2x80x9c2xe2x80x9d. The laser beam required to scan the information is denoted by the reference numeral xe2x80x9c7xe2x80x9d.
Accordingly, the construction of DVDs differs from that of the generally known CDs (compact discs) in the fact that (DVDs have a sandwich construction) Whereas CDs essentially consist of a 1.2 mm thick disc of polycarbonate or poly(meth)acrylate resin, (DVDs are made of two 0.6 mm thick discs for which polycarbonate is almost exclusively used today.) Through a refined data structure and lasers of minimal wavelength, the information layer of a DVD can contain ca. 4.7 gigabytes of information whereas conventional CDs can only store around 640 megabytes of information.
The sandwich construction of DVDs means that the two layers 1 and 2 have to be joined together. Originally, solvent-based adhesives were used to join the two layers together. More recently, hotmelt adhesives, UV-curable solvent-free liquid adhesives and UV-crosslinkable hotmelt adhesives have been proposed.
The following steps are absolutely essential in the production of a DVD:
The polycarbonate or poly(meth)acrylate blanks have to be made by injection molding.
The blank 1 carrying the layer of information is coated with a reflective layer. This is generally a reflection layer, for example of aluminium, applied by vapor deposition in vacuo.
The reflective layer has to be protected against corrosion immediately after production. In one known embodiment, therefore, a lacquer film cured by UV radiation is applied to the reflective layer. An alternative method comprises applying a protective film of a thermoplastic material.
The blank 2 without a layer of information can be printed with graphics and text by various methods.
The blanks 1 and 2 are bonded together with an adhesive.
DE-A-3224647 describes a process for the production of optical video discs or digital audio discs. According to this document, the layer carrying the information, after coating with a reflective film of aluminium, is provided with a protective film of a thermoplastic film-forming material which melts at a predetermined temperature and hardens at room temperature. In one embodiment, this thermoplastic film-forming material is also the adhesive for bonding the two disc substrate halves together. In another embodiment, this film-forming material is coated with another adhesive which is tacky at room temperature. The two disc substrates are bonded together with this adhesive. DE-A-3224647 does not provide any details of the composition of the thermoplastic film-forming material or of the adhesive tacky at room temperature. In addition, there is nothing in the teaching of DE-A-3224647 to indicate whether these adhesive materials would also be suitable for the production of DVDs.
DE-A-3246857 describes an optical disc comprising a pair of substrates of which at least one has information pits formed on one surface. A metallic layer of a reflective film is applied to this layer of information pits and a protective resin layer is in turn applied to the metallic reflective film. Applied to the protective resin layer is a layer of adhesive which is used to join the two substrate halves. According to DE-A-3246857, hotmelt adhesives containing a mixture of one or more thermoplastic elastomers as basic polymer are used as the adhesive. In addition, it is clear from the teaching of DE-A-3246857 that the hotmelt adhesives described therein contain normal tackifying resins and additives, for example fillers, antioxidants or UV absorbers, to increase their heat and weathering resistance. It is also stated that the melt viscosity of the adhesive should not exceed a value of 1,000 poises at 160xc2x0 C. to ensure that the adhesive does not have any adverse effects on the metallic film applied by vapor deposition. There is no mention of the suitability of these adhesives for the production of DVDs.
According to the teaching of DE-A-3840391, the use of hotmelt adhesives for bonding substrate pairs in the production of video discs or digital audio discs is problematical on account of their poor thermal stability. The use of UV-curable monomer compositions as the adhesive can cause corrosion of the substrate. For bonding the substrate pairs in the production of video discs, DE-A-3840391 proposes the use of UV-crosslinkable hotmelt adhesives. There is nothing to indicate whether these UV-crosslinkable hotmelt adhesives would be suitable for the production of DVDs.
As mentioned at the beginning, DVDs are distinguished from known CDs or optical discs (also known as laser discs) by a high data or information storage density of the storage medium so that they impose greater demands on the production process and on the materials used therein, for example adhesives. The associated problems are described in detail in EP-A-0 735 530. In view of the high information density and the small diameter of a disc of a DVD, the tolerable deviation from the optimal planar orientation of the disc on insertion into the player is significantly smaller than with conventional laser discs so that a minor deviation of the orientation of the disc can clearly falsify the information signal reproduced. For this reason, a DVD inserted into the player has to be able to stay in the player without warpage or distortion, even under the thermal stress of a relatively long playing time, although the two individual layers of the DVD only consist of about 0.6 mm thick plastic blanks. UV-curing adhesives tend towards adhesion failures, for example when subjected to impact stress, because they are generally very brittle after curing. Also, most UV-curing adhesives and other two-component adhesives are characterized by relatively high inherent shrinkage. The resulting force applied by the adhesive to the reflective layer can impair the information content and operational reliability of the DVD. For this reason, EP-A-0 735 530 proposes a combination of a hotmelt adhesive and a liquid two-component adhesive for bonding the two halves of the DVD together. Compositions for the proposed hotmelt adhesives are not mentioned. It is proposed that cationically polymerizing UV-curing adhesives be used for the liquid two-component adhesives.
WO-A-98/40833 describes the use of a hotmelt adhesive with a melt viscosity of more than 100,000 mPa.s at 160xc2x0 C. for bonding digital video, discs (DVDs) of sandwich construction. In preferred embodiments, the hotmelt adhesives are said to contain pigments to obtain high-contrast backgrounds for improving the images/graphics and text on the DVDs. WO-A-98140833 does not provide any particulars of the thermal stability of the DVDs thus bonded, particularly with regard to distortion-free scanning of the information stored on the DVD.
JP-A-09 208 919 describes compositions of hotmelt adhesives for optical discs which consist essentially of three components. The first component is a styrene resin consisting of styrene/diene block copolymers or hydrogenated derivatives there of. The second component of the hotmelt adhesives is a tackifier based on rosin, terpene or petroleum resins, the softening point of the tackifier being said to be at least 115xc2x0 C. and preferably at least 140xc2x0 C. In addition, the hotmelt adhesives according to the teaching of the document in question are said to contain waxes, preferably paraffin waxes, micro waxes, low molecular weight polyethylene, low molecular weight polypropylene and non-crystalline poly-xcex1-olefin. In particular, it is proposed to use waxes with no functional group in their molecular structure, preferably crystalline polypropylene with an average molecular weight of at most 20,000. Although JP-A-09 208 919 does mention standard adhesive properties, such as ring-and-ball softening point, melt viscosity, peel strength, bond strength and creepage at elevated temperature, the information provided is not conclusive as to the suitability of these adhesives for bonding DVDs.
Among the latest advances in the quality control of optical data carriers, above all DVDs, test systems have recently been developed to enable the tilt and dishing of DVDs to be determined by triangulation. To this end, the deflection of a reflected laser beam is evaluated. The degree of deflection is recorded through a receiver, which converts the position of the deflected beam into electrical signals, and transmitted to the analog/digital converter for evaluation. One example of such a test system is the Optical Disc Test System Advanced Version (ODT-A) of Conttec GmbH.
The adhesive used influences both the tilt and the dishing of a disc to a considerable extent, particularly in the presence of heat and/or: moisture.
Against the background of this prior art, the problem addressed by the present invention was to provide an adhesive which would enable DVDs to be economically produced without the disadvantages of UV-curing adhesives or the hitherto known hotmelt adhesives and which would still meet the strict requirements of the latest quality control standards for DVDs.
The solution to this problem is defined in the claims and consists essentially in the use of a hotmelt adhesive for the production of digital versatile discs (DVDs) based on
a) at least one thermoplastic elastomer,
b) at least one hydrocarbon resin,
c) at least one poly-xcex1-olefin,
d) at least one polar wax bearing functional groups.
In. addition to the key components mentioned above, the hotmelt adhesives to be used in accordance with the invention may also contain tackifying resins, optionally plasticizers, stabilizers/antioxidants, optionally fillers or extenders, pigments, coupling agents and mixtures therof.